Optical fibers have become increasingly important as a medium for transmitting large quantities of information in the form of lightwaves. There are a number of ways of making optical fiber, but most require the deposition of glass soot from a flame, a process known generally as flame pyrolysis or flame hydrolysis.
One glass deposition method, referred to as Outside Vapor Deposition (OVD), involves deposition of glass soot by flame pyrolysis on a mandrel to form a hollow cylindrical porous soot form of glass particulate. The deposited porous soot cylinder is then consolidated into a glass substrate tube by heating the soot cylinder, which is mounted on a mandrel, in a furnace. As described, for example, in the copending application of T. J. Miller, Ser. No. 522,613, filed May 14, 1990, hereby incorporated herein by reference, a glass preform can be made from the glass substrate tube by vapor depositing glass on the inner surface of the tube and then collapsing the entire structure. The collapsed structure can be used directly as a preform or it can be enlarged by inserting it into another tube, called an overclad tube, in accordance with the "rod-in-tube" approach, as described, for example, in the patent of J. W. Baumgart, et al., U.S. Pat. No. 4,4820,322, granted Apr. 11, 1989. The optical fiber is made by using glass drawing to pull the fiber from the heated and softened finished preform.
Another deposition method for making glass is the Vapor-phase Axial Deposition (VAD) method, as described, for example, in the patent of S. E. Miller, U.S. Pat. No. 3,966,446, granted Jun. 29, 1976, and in the patent of Kawachi et al., U.S. Pat. No. 4,345,928, issued Aug. 24, 1982. Glass soot is deposited from a flame onto a bait rod, which is rotated and slowly moved away from the flame resulting in a substantially solid soot cylinder. After consolidation, the solid structure may be used as a preform or, as before, enlarged by the rod-in-tube process to make the final preform.
In any method that is used, the cladding or outer surface portion of the final optical fiber must have a lower refractive index than that of the core, and this is normally accomplished by including impurities in either or both the clad region and/or the core region to modify the refractive index of the glass. The U.S. patent of Miller et al., Ser. No. 459,605, filed Jan. 2, 1990, for example, shows the inclusion of a fluorine constituent in the depositing flame for doping the deposited glass soot with fluorine, which tends to reduce the refractive index of the glass. The patent of D. W. Monroe et al., U.S. Pat. No. 4,915,716, granted Apr. 10, 1990, for example, describes a germanium constituent in the flame, which has the effect of increasing the refractive index. Flame pyrolysis, whether or not it includes a doping constituent, has thus become virtually indispensable for making high quality glass to be used as preforms, substrate tubes or overclad tubes.
From the foregoing, one can see that making glass by flame pyrolysis with subsequent consolidation of the deposited glass soot is used in a number of different ways to fabricate the preforms from which optical fiber may be drawn. The use of optical fiber as a transmission medium has become so widespread that it is now considered a commodity product, and great efforts are being made to achieve small reductions in the cost of making it. If one could increase the rat at which the glass soot is deposited by the flame pyrolysis method, it is recognized that one could reduce the cost of the optical fiber product.